The invention is in the field of yogurt production by fermentation. Fermentation is a well-known technique for the production of yogurt using the metabolic activity of microorganisms that release acid.
Microorganisms that release acid are well-known to be used in a feed culture comprising milk, resulting in yogurt that has a longer shelf-life than milk. Examples of well-known acid-releasing microorganisms for use in yogurt production are microorganisms from the genus: Lactobacillus, Lactococcus and Streptococcus. 
In a typical yogurt fermentation process, three phases can be distinguished. The first phase starts when the microorganisms are combined with the fermentation feed, usually a milk-based feed. The microorganisms adapt to their new environment, and start to take up nutrients, such as peptides, amino acids, vitamins and minerals. In this phase, the microorganisms produce enzymes required for cell division and growth, for spending energy, and for making storage materials, building blocks or nutrients. In this phase, however, there is barely microorganism growth, or any other visual indication that anything is happening in the fermentation. For this reason, this phase is called the lag phase.
The lag phase is characterized in that the presence of certain nutrients may be the limiting factor for growth. An example is a system in which the amount of peptides is insufficient to allow for normal microorganism growth or a normal microorganism growth rate. As long as peptide presence remains insufficient, growth remains limited by the peptide concentration. Even though it appears nothing happens, this phase is very important for the fermentation process because the health of the population of microorganisms determines the quality of the resulting yogurt.
When the microorganisms have adapted to their environment, the second phase initiates. This phase, characterized by a non-substrate limited microorganism growth, is called the exponential phase. During the exponential phase, the microorganisms start to grow by cell division, and therefore multiply exponentially. In this phase, the microorganisms as a consequence of their metabolic character produce among others lactic acid.
At the end of the exponential phase, the amount of suitable nutrients has often decreased such that exponential growth can no longer be sustained by the fermenting milk mixture. Thus, growth slows down and the fermentation enters the stationary phase. In this phase, growth is no longer exponential, although cell division still occurs, and the fermenting mixture slowly attains an equilibrium between all present compounds. If all circumstances are appropriate, this results in a yogurt product of high quality, with well-balanced flavor and smell.
The time these stages require is highly variable, and dependent on the type of microorganism(s) used, the type of fermentation feed, the temperature and many other parameters. Given these distinct phases, production of yogurt is commonly a batch process. As is common for batch processes, an important factor in cost is the time required for the product to be ready.
An important factor in production time is the lag phase. During this phase, the actual fermentation process is prepared. Apart from creating the adequate conditions for microorganism growth, there is no contribution at all to the making of the product of interest, and as such, a shorter lag phase would have a huge impact on the economy of the fermentation process. However, the lag phase is very important for determining the health of the population of microorganisms, which in turn is important for the quality of the yogurt. The time that is required for the lag phase to pass and the fermentation process to reach the exponential phase is referred to as the lag time.
Attempts to reduce the lag time have been made before. One option is to use a semi-continuous fermentation process, in which the microorganisms are adapted to the production stage and remain in the exponential phase for a prolonged time. This however, is often not suitable, because the stationary phase is important for determining the final taste and/or quality of the yogurt, and this phase is bypassed in such a semi-continuous process.
Also, it is possible to a d a mix of microorganisms, called a starter culture, which have already been adapted to the medium conditions of the fermentation. This, however, creates different problems, because in a small-scale premix microorganism feed, the environment of the full-scale fermentor is difficult to copy. It is possible to use a larger volume of the preculture (inoculum), but this has a big impact on the production process and costs of the preculture stage. Therefore, it would be preferred to reduce the lag time, possibly even further than possible with this technique, in a reliable way, with a limited amount of starter culture.
For reducing the lag time, it is also possible to add extra easily transportable and energy beneficial nutrients to the premix, as for instance extra peptides. However, this creates additional costs and problems with for instance off-taste and coloring.